Comparative removal of hazardous cationic dyes by MOF-5 and modified graphene oxide

Among cationic dyes, malachite green (MG) is commonly used for dying purposes and also as an inhibitor in aquaculture, food, health, and chemical industries due to its cytotoxic effects. Therefore, MG removal is essential to keep the ecosystem and human health safety. Adsorption is a viable and versatile option and exploring efficient adsorbents have high priority. Herein, MOF-5 and aminated corn Stover reduced graphene oxide (ACS-RGO) of typical adsorbents of metal–organic-frameworks (MOFs) and carbon-based classes were studied for MG removal. MOF-5 and ACS-RGO had a specific surface area and total pore volume of 507.4 and 389.0 m2/g, and 0.271 cm3/g and 0.273 cm3/g, respectively. ACS-RGO was superior for MG adsorption and the kinetic rate coefficient for ACS-RGO was ~ 7.2 times compared to MOF-5. For ACS-RGO, MG removal remained high (> 94%) in a wide range of pH. However, dye removal was pH-dependent for MOF-5 and increased from ~ 32% to ~ 67% by increasing pH from 4 to 12. Increasing dye concentration from 25 mg/L to 100 mg/L decreased adsorption by MOF-5 and ACS-RGO for ~ 30% and 7%, respectively. Dye removal was evident in a few tens of seconds after adding ACS-RGO at doses above 0.5 g/L. A significant loss of 46% in adsorption was observed by decreasing MOF-5 mass from 1 to 0.1 g/L. ACS-RGO removed MG in multilayer with an exceptional adsorption capacity of 1088.27 mg/g. In conclusion, ACS-RGO, and MOF-5 showed promising kinetic rates and adsorption capacities toward MG.

www.nature.com/scientificreports/ Among these methods adsorption is a favored technique due to the numerous advantages it has over different methods available for pollution control. Easy design and operation, environmental benign nature, high efficiency under low pollutant concentration, availability of adsorbents, the flexibility of the process make adsorption an interesting and versatile option. Numerous studies oriented in the past years to develop new structures to maximize the advantages of adsorbent materials 12 . In recent years, many materials have been used as adsorbents in industrial dyed wastewater treatment, for example agricultural waste 12 , natural mineral 13 , activated carbon 14 , zeolite 15 , Graphene oxide 16 and Metal-organic-frameworks 17 . Metal-organic-frameworks (MOFs) are an exceptional class of porous materials composed of inorganic (metal ions or clusters) and organic (linker or ligand) components. MOFs are highly crystalline with a huge internal surface area and pore volume. Furthermore, they are highly tunable to design and functionalize, and hence they are recognized as materials of interest in applied sciences 18 .
Graphene is the thinnest material produced so far and the simplest form of carbon that exists in two-dimensional form. Due to the exceptional mechanical, electrical, thermal, biological, optical, and other physicochemical properties of graphene and its derivatives, the graphene family experienced massive and rapid growth in the fields of electronics, biosciences, and environment 26 .
Graphene oxide (GO) is a scalable derivate of graphene that is highly hydrophilic and rich indifferent functional groups such as carboxyl, hydroxyl, and epoxy. Reduced graphene oxide (RGO)is also a type of graphene that the presence of abundant hydroxyl functional groups has caused the hydrophobicity significantly 27,28 . The presence of functional groups and the adaptability of being composite, functionalized, and decorate, surged interest toward graphene family for treatment purposes such as adsorption, catalysis, membrane separation, ion exchange, dialysis, etc. 29,30 . As an adsorbent, graphene family has been studied and promising results obtained for the removal of heavy metals 31 , pesticides 32 , antibiotics 33,34 , and other emerging contaminants. The aforementioned functional groups on graphene surface play as active sites for modification by basic moieties like amines. In some cases, GO needs more modification to improve the adsorptive properties for target contaminants. RGO, on the other hand, has a simpler and more predictable behavior for surface modification because it contains only one type of functional group. Of the variety of techniques proposed for amine functionalization such as plasma electron beam, hydrothermal reaction, and Leuckart reaction, the hydrothermal approach is most common. Fanget al prepared GO-NH 2 Nano sheets with a surface area of 320 m 2 /g and rapid adsorptive properties for cobalt cations 35 . Awad et al. improved GO adsorption properties for mercury (II) through the incorporation of different functional groups using solvothermal methods. They reported a premium removal of 100% when GO was functionalized by carboxylic acid 36 . Viana et al. functionalized graphene oxide with diethylenetriamine using a microwave-assisted method and then prepare a hydrogel adsorbent. The prepared hybrid hydrogel was finally used as an efficient adsorbent for methylene blue 37 . In the previous study, we reported a green approach for valorizing corn Stover biomass to reduced graphene oxide (RGO). A post-treatment approach was then applied to convert RGO to aminated graphitic carbonaceous structure (ACS-RGO). Afterward, ACS-RGO was used as a promising adsorbent for antibiotic tetracycline where a high 132.9 mg/g adsorption capacity was obtained at pH 7.4 38 .
In this regard for the first time, the goal was to evaluate the dye adsorption properties of MOF-5 as a representative of hybrid structures and ACS-RGO as a typical member of carbonaceous materials. Therefore, this study was initiated by a comparative analysis for adsorption efficiency and rate of dye removal by kinetic modeling. The study then oriented toward the detailed analysis of the effects of operating variables i.e. pH, adsorbent dose, MG concentration, and mixing time. To elucidate the mechanism of adsorption, and to compare the adsorption capacity of herein materials, the equilibrium of the adsorption system was investigated.

Materials and methods
Chemical and reagents. Chemicals used in the study were of analytical grade and purchased from Merck and Sigma Aldrich companies. Adsorbate, malachite green oxalate = Basic Green 4, chemical formula = C 52 H 54 N 4 O 12 , MW = 927, the wavelength of maximum absorbance = 624 nm was purchased from Sigma Aldrich. Terephthalic acid (TPA) = C 8 H 6 O 4, molar mass = 166.13 g/mol, N,N'-dimethylformamide (DMF), and zinc nitrate hex hydrate were used without further modification.
Adsorbents preparation and characterization. MOF-5 was prepared by the protocol described in the literature 39 . In brief, 0.595 g zinc nitrate hexahydrate and 0.111 g TPA were dissolved in 20 mL DMF. The clear solution was then sonicated at 35 kHz for 10 min and transferred to a Teflon-lined autoclave where it was heated for 24 h at 135 °C. After cooling at room temperature, the white precipitates were collected by centrifuge and washed with fresh DMF, and dried overnight.
A detailed protocol for the synthesis of aminated corn Stover-based reduced graphene oxide (ACS-RGO) including collecting Agro waste materials, washing, calcination, activation, thermo-chemical treatment, and surface modification with amine was described in our previous work 38 .
The presence of functional groups on the surface of the adsorbent were ascertained by recording Fouriertransform infrared spectroscopy (FTIR) by a Thermos Nicolet, Avatar 370 spectrophotometer.
The surface morphology and crystal texture were studied by field emission scanning electron microscopy (FE-SEM) using MIRA3 TESCAN, Czech Republic. The structure of crystals was analyzed by X-ray diffraction www.nature.com/scientificreports/ (XRD) usingUnisantis S.A, XMD300 model, Geneva, Switzerland, with Cu-kα as source radiation at wavelength 0.154 nm), over the range of 10° to 80°. Pore volume, specific surface area (SSA), and pore sizes of adsorbents were examined by the nitrogen sorption using BELSORP-mini-II (BEL Japan, Inc.).
Adsorption experiments. The present study was designed to compare the dye adsorptive characteristics of porous materials from two interesting classes, MOFs and carbonaceous structures. When MOF-5 and ACS-RGO were synthesized and characterized, the experimental study began with a comparative analysis of MG removal using adsorption capacity and kinetic modeling. The kinetic parameters for dye removal were estimated by non-linear regression models. Next, parametric evaluation and equilibrium modeling were conducted. Effect of pH (4-12), initial dye concentration (25-100 mg/L), mixing time (2-60 min), and adsorbent dose (0.1-1 g/L) were surveyed in parametric evaluation step.
All the experiments were performed in batch mode, at room temperature, and under mixing at 250 rpm. After adsorption, materials were separated by centrifuge and dye concentration in supernatants determined by UNICO UV-2100 spectrophotometric method at 626 nm. As presented in Fig. S1, it is noticeable that the light adsorption intensity for MG is pH-dependent and adsorption intensity was reduced over 50% by increasing pH from 4 to 12. Fig. S1 shows the dye intensity for a solution containing 50 mg/L MG at pH 4 and pH 12. Dye removal efficiency (µ%) was calculated for each run by the difference between dye concentration after (C, mg/L) and before adsorption (C 0 , mg/L): The capacity of adsorbents in any time (q t , mg/g) was calculated according to the mass of adsorbents in the solution (m, g), the volume of solution (V,L), initial dye concentration (C 0 , mg/L) and dye concentration at any time (C t , mg/g) 40 : Adsorption modeling. Adsorption modeling is a suitable approach for obtaining basic information required to scale up the system. Kinetic models describe the rate of adsorption system and are insightful in identifying the rate-limiting step in the process. Isotherm models, on the other hand, describe the equilibrium state of the sorption and provide a useful tool for comparing adsorbents for a specific contaminant and also the utilization rate of adsorbents in real treatment systems. Kinetic data was collected by performing the adsorption experiments at different mixing times. Three common non-linear models i.e. pseudo-first-order (PFO), pseudosecond-order (PSO), and intraparticle diffusion model (IDM) were fitted to the kinetic data.
The equilibrium data was collected by conducting adsorption experiments on solutions withthe different initial dye concentrations in the range of 50-300 mg/L. The adsorbent capacities were then calculated and modeled using Langmuir, Freundlich, and Javanovich models.

Regeneration study of ACS-RGO.
To conduct the regeneration tests, the well saturated ACS-RGO was contacted with 0.1 M hydrochloric acid solution (0.1 mol/L HCl) as eluting agent. The adsorbed MG dye desorbed under 2 h' agitation at 250 rpm. After separating ACS-RGO, the MG concentration in the supernatant was measured. ACS-RGO then rinsed with distilled water twice and used for the next MG removal cycle under optimal conditions. MG desorption ratio was determined using the following equation:

Results and discussion
Adsorbent characteristics. The FESEM images in Fig. 1 (left) show the morphology of MOF-5 crystals.   FTIR spectra for MOF-5 in Fig. 3 show chemical fingerprint peaks of this material at around 1580-1590 cm −1 which is indicative of asymmetric and symmetric stretching vibrations of -COO-in TPA linker. Also, the sharp peak at around 1506 cm −1 can be attributed to C=C stretching vibration in the linker. The graph also shows two  Table 1summarizesthe information related to the BET specific surface area (SSA), total pore volume, and mean pore diameter(nm) for MOF-5 and ACS-RGO. SSA is an important factor related to surface phenomena such as adsorption. The synthesis condition, source of inorganic metals, the ratio of metal/linker, type of solvent, Kinetic study. In the screening analysis, the efficacy of MOF-5 and ACS-RGO for dye removal was determined using adsorbent capacity and kinetic parameters. The experiments were conducted in the presence of 0.4 g/L of adsorbent in solutions containing50 mg/L MG. Dye removal was monitored for up to 60 min. The data modeled by non-linear kinetic models are described elsewhere 52 and the results are shown in Fig. 4 and Table 2.
As seen, the kinetic rate constants of MG removal by ACS-RGO was by far higher than those for MOF-5. Adsorbent capacity for MOF-5 and ACS-RGO after 60 min contact time were estimated68.6 mg/g and 123.8 mg/g, respectively. Hence, ACS-RGO was superior for MG removal compared to MOF-5. Moreover, according to sta-    www.nature.com/scientificreports/ tistical parameters i.e. a higher R 2 Adj and R 2 , and a lower residual sum of squares (RSS) and reduced Chi-Sqr, the kinetic of MG adsorption for both adsorbents fitted well by the pseudo-first-order model. The higher adsorption for ACS-RGO could attributed to the low pH.
Parametric study. Having information on the effect of operating variables is critical in optimizing the process for the highest efficiency. In adsorption systems, the pH of the solution, the concentration of pollutants, and adsorbent dose are important variables to be studied. The effect of pH in the range of 4 to12, MG concentration in the range of 25-100 mg/L, and adsorbent dose in the range of 0.1-1 g/L were studied and the results are shown in Fig. 5(a-c). As seen, in all cases, the efficacy of ACS-RGO is by far higher than MOF-5 for MG removal.
pH is an important factor governing the adsorption system through affecting the adsorbent and adsorbate charge, the level of hydroxyl and hydrogen ions, and also the charge of co-existence species in the aqueous environment. Figure 5(a) shows the effect of solution pH on dye removal by MOF-5 and ACS-RGO. The figure illustrates that the increasing pH from 4 to 12 improved MG removal by both adsorbents. However higher pH is favor for dye removal by ACS-RGO, the removal efficiency remained high (> 94%) in a wide range of pH from 6 to 12. For MOF-5, MG removal increased from ~32 to ~67% by increasing pH from 4 to 12. As a cationic dye, MG exists in aqueous environments in cationic form. The elevated dye adsorption by pH could be attributed to   www.nature.com/scientificreports/ the surface charge of adsorbent materials and MG ionic form. pH for MOF-5 and ACS-RGO were 4.6 and 8.3, respectively. The surface charge of adsorbents turns negative at pH values above the pH, and hence a predominant electrostatic attraction force enhances the MG removal. Incremental removal by pH was observed in MG removal by chemically modified rice husk 53 , natural zeolite 54 , and reduced graphene oxide 55 . In some of these studies, removal percentage keeps constant at pH values over an optimum level. Dye concentration is also an important factor determining the adsorption efficiency. Figure 5(b) shows adsorption removal was dye concentration-dependent for both adsorbents. The removal efficiency for MOF-5 and ACS-RGO decreased from ~ 71% to about 40% and from ~ 99% to about 92% by escalating MG concentration from 25 to 100 mg/L. The high affinity of ACS-RGO causes a premium dye uptake even at concentrated solutions. Lower adsorption rate at high concentrations is attributed to the competition between MG ions for infinite adsorption sites on the surface. Adsorption may also hinder by limitation in mass transfer rate at the higher dye concentration. It is noticeable that contrary to adsorption efficiency, the adsorbent capacity increased dramatically by dye concentration in the studied range, from ~ 44 to ~ 101 mg/g, and from ~ 62.4 to ~ 231 mg/g for MOF-5 and ACS-RGO, respectively. Similar observations were reported for dye eriochrome black-T removal onto ZIF-67-OAc 56 , caffeine removal by oxidized carbon 57 , Cr(VI) removal by fly ash 58 , antibiotics 59  www.nature.com/scientificreports/ MOFs, direct Blue-71 removal onto multi-walled carbon nanotubes 60 , and in advance oxidation processes such as petroleum hydrocarbons degradation by ozonation 61 . Mass of adsorbent applied to the system is another important variable that provides the removal sites for the sorbate. Dose of MOF-5 and ACS-RGO between 0.1-1 g/L was investigated and the results are shown in Fig. 5(c). MG removal by ACS-RGO was almost complete at doses above 0.4 g/L and the removal decreased to ~ 89% by decreasing dose to 0.1 g/L. Interestingly, the dye removal occurs rather fast in a few tens of seconds after adding ACS-RGO at doses above 0.5 g/L. Dye adsorption by MOF-5 happened slowly and removal efficiencies decreased from ~ 72% to ~ 26% by decreasing mass from 1 to 0.1 g/L. Increased adsorption by dose was reported for heavy metals removal by low-cost biosorbents 62 .
Isotherm modeling. Adsorption isotherms are mathematical models describing the system behavior when the adsorbate/adsorbent reaches the equilibrium state. These mathematical models are useful tools to estimate basic parameters for the design and operation of real adsorption units. In this study the equilibrium data were obtained experimentally by performing the experiments at pH = 10, ACS-RGO dose = 0.2 g/L, MOF-5 = 1 g/L, mixing time = 120 min, and five different initial MG concentrations varied from50-300 mg/L. The data first fitted to linear form to have an estimation of isotherm parameters. Since non-linear regression is preferable and gives a more accurate estimation of model parameters 63 , they applied to the experimental equilibrium data.
The non-linear form of isotherm models that are of two parametric classes are given in Table S1 52 . The illustration of isotherm models and values obtained for adsorbents are presented in Fig. 6 and Table 3, respectively.
The maximum monolayer adsorption capacity (qmax) estimated by the Langmuir model is a useful tool to compare the economic feasibility of different adsorbents toward a specific contaminant. The qmax values for MG  www.nature.com/scientificreports/ reported for carbonaceous materials and MOF-based adsorbents are presented in Table 4. As seen, the ACS-RGO is superior to many reported carbon-based adsorbents. Present study highlighted the significant role of surface modification of carbon-based materials to improve their adsorptive properties. The adsorption capacity of MOF-5, on the other hand, was not significant among the studied MOF-base materials. A challenging issue in the application of some MOFs is the structural stability of these materials in aqueous environment. This is a case for MOF-5 that has a metastable structure in water medium. Therefore, a sample of a MOF may have a high adsorption capacity, but during the adsorption and as a result of agitation dissolved gradually and leached the adsorbed dye into the solution. Figure 7 present the result of four series of regeneration/reuse of ACS-RGO. Dilute hydrochloric acid was used as the eluting agent to desorb MG. As seen in the Fig. 7, after four cycles of regeneration and reuse, the adsorption capacity of ACS-RGOMG reduced only 6.1%. The insignificant loss in removal efficiency proved that the predominant mechanism for MG removal by ACS-RGO was being ion exchange. Moreover, the reusability of ACS-RGO indicated the promising nature of ACS-RGO to alleviate the nuisances of dyes in the environment.

Conclusion
In this study, two adsorbent materials, i.e. MOF-5 and aminated corn Stover reduced graphite oxide (ACS-RGO) of typical adsorbents of metal-organic-frameworks (MOFs) and carbon-based classes were studied for cationic MG adsorption. MOF-5 and ACS-RGO had a specific surface area and total pore volume of 507.4 and 389.0 m 2 /g, and 0.271 cm 3 /g and 0.273 cm 3 /g, respectively. ACS-RGO was superior for MG adsorption and the kinetic rate and adsorption capacity for ACS-RGO was ~ 7.2 and ~ 21 times compared to MOF-5. For ACS-RGO, MG removal remained high (> 94%) in a wide range of ph. Dye removal onto MOF-5 increased from ~ 32% to ~ 67% by increasing pH from 4 to 12. Increasing dye from 25 mg/L to 100 mg/L decreased adsorption by MOF-5 and ACS-RGO for ~ 30% and 7%, respectively. Dye removal was rather fast and significant removal was observed in a few tens of seconds after adding ACS-RGO. Multilayer adsorption with a huge adsorption capacity of 1088.27 mg/g described MG adsorption on to ACS-RGO.

Data availability
The datasets generated and analysed during the current study available from the corresponding author on reasonable request.